Powder coating composition comprising epoxy group-containing acrylic resin

ABSTRACT

An epoxy group-containing acrylic resin for a powder coating composition is a copolymer, having a weight-average molecular weight of 3000 to 20000, of a vinyl monomer mixture comprising 10 to 60% by weight of alkyl ester(s) of (meth)acrylic acid, 10 to 60% by weight of epoxy group-containing vinyl monomer(s) and other vinyl monomer(s), said epoxy group-containing acrylic resin having a mean particle diameter in the range of 80 to 800 μm, including 95% by weight or more of particles satisfying the relation of 1≦R/r≦1.2 wherein R is a long diameter and r is a short diameter of each particle, and including 1000 ppm or less of volatile components, the amount of the particles having long diameters R of less than 62 μm being 5% by weight or less, that of the particles having short diameters r of more than 1000 μm being 5% by weight or less.

This application is a divisional of U.S. application Ser. No.09/114,627, filed Jul. 13, 1998, abandoned, which is a continuation ofinternational Application No. PCT/JP97/00032 filed on Jan. 10, 1997which designates the United States pursuant to 35 U.S.C. §363.

TECHNICAL FIELD

The present invention relates to an acrylic resin for a powder coatingcomposition and its preparation method.

BACKGROUND ART

Examples of coating compositions include a solvent type coatingcomposition in which a pigment and a polymeric substance are dispersedin an organic solvent, a water-soluble type resin coating composition inwhich a water-soluble resin that can be insolubilized by heating isused, an emulsion coating composition in which a resin is dispersed inwater, and a powder coating composition in which a resin is ground intopowder, applied and then heated to form a coating film, but the solventtype coating composition has been mainly used from the viewpoints of thebeauty of an obtained coating film, durability and the like.

In recent years, however, regulations for organic solvents become severein view of problems such as air pollution and the danger of a fire, andmuch attention has been paid to solventless coating compositions. Aboveall, the powder coating compositions have been in the limelight from theviewpoint of environmental protection, and in particular, acrylic powdercoating compositions are characterized by being excellent in a coatingfilm performance such as weathering resistance. For this reason, it hasbeen noticed to use these acrylic powder coating compositions forbuilding materials and car parts, particularly for clear coatings forcars.

With regard to the powder coating composition, in order to form a smoothcoating film, it is necessary to lower the molecular weight of aselected resin. For the sake of this realization, as preparation methodsof the resin for the powder coating composition, there have beensuggested bulk polymerization, solution polymerization, suspensionpolymerization and the like, but in fact, the solution polymerizationmethod in which the molecular weight can be easily lowered is oftenemployed (Japanese Patent Application Laid-Open No. 138437/1978).

However, in the solution polymerization method, a step of removing aused solvent is required, and for the removal of the solvent, theconditions of a high temperature and a reduced pressure are usuallynecessary, which is inconvenient in point of energy. In addition, aproduction process is prolonged, which inconveniently results in poorproductivity. Furthermore, in the solution polymerization method, theresin is obtained in the state of masses, and in order to obtain thecoating composition, a grinding step of the resin is necessary. However,when the resin is ground, fine powder is easily generated, and such finepowder is liable to fly about during handling, which easily makes anoperating efficiency poor. Moreover, if the insufficiently ground resinis present, the mixing of the resin and a crosslinking agent isinsufficient during the preparation of the powder coating composition,which causes a problem that the performance of the obtained coating filmdeteriorates. Furthermore, a method for preparing an acrylic resin forthe powder coating composition by the bulk polymerization undersolventless and catalyst-free conditions has been suggested in JapanesePatent Application Laid-Open No. 140395/1978, but also in this bulkpolymerization, the grinding step of the resin is necessary, so that thebulk polymerization has the same problem as in the case of the solutionpolymerization.

On the other hand, the suspension polymerization has an advantage thatthe solvent removing step and the resin grinding step are unnecessary,but there is a tendency that the molecular weight of the obtained resinis very high. In consequence, the gloss and the smoothness of the formedcoating film are inconveniently easily impaired. Thus, a mercaptan hasheretofore been used as a chain transfer agent to lower the molecularweight. However, a powder coating composition comprising the resinobtained by the use of the mercaptan as the chain transfer agent is soodorous as to be impractical, and a formed coating film is apt to bepoor in weathering resistance.

On the contrary, in Japanese Patent Application Laid-Open No.158682/1975, it has been disclosed that for the purpose of obtaining alow-molecular weight vinyl polymer which is suitable for the powdercoating composition and is less odorous by the suspensionpolymerization, α-methylstyrene and/or its dimer is used as the chaintransfer agent and as an essential component.

However, a coating film formed from the vinyl polymer obtained inJapanese Patent Application Laid-Open No. 158682/1975 is still poor incoating film performance such as smoothness. Particularly, since a largeamount of the chain transfer agent is used, a polymerization ratedeteriorates, so that a polymerization step is prolonged. Inconsequence, the productivity of the resin is low, and when the resin ismolten and kneaded at a high temperature in the preparation of thepowder coating composition, an odor is given off owing the increasedamount of the remaining monomer. In addition, during the formation ofthe coating film, vaporization of the remaining monomer occasionallycauses the formation of pinholes in the coating film.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a resin for a powdercoating composition which is excellent in workability, productivity andcoating film performances such as gloss and smoothness and which is freefrom the problems of an odor and pinholes.

Another object of the present invention is to provide a method forefficiently preparing the resin for the powder coating composition.

The present inventors have intensively investigated with the intentionof achieving the above-mentioned objects, and as a result, it has beenfound that an epoxy group-containing acrylic resin having a specificparticle shape and specific physical properties can achieve theabove-mentioned objects, and in consequence, the present invention hasbeen completed.

That is to say, the present invention is directed to an epoxygroup-containing acrylic resin for a powder coating composition which isa copolymer, having a weight-average molecular weight of 3000 to 20000,of a vinyl monomer mixture comprising 10 to 60% by weight of alkylester(s) of (meth)acrylic acid, 10 to 60% by weight of an epoxygroup-containing vinyl monomer(s) and other vinyl monomer(s), said epoxygroup-containing acrylic resin

having a mean particle diameter in the range of 80 to 800 μm,

including 95% by weight or more of particles satisfying the relation of1≦R/r≦1.2 wherein R is a long diameter and r is a short diameter of eachparticle,

the amount of the particles having long diameters R of less than 62 μmbeing 5% by weight or less, that of the particles having short diametersr of more than 1000 μm being 5% by weight or less, and

including 1000 ppm or less of volatile components.

In the present invention, it is required that an epoxy group-containingacrylic resin, which can be used as a resin for a powder coatingcomposition of the present invention, has a mean particle diameter inthe range of 80 to 800 μm and includes 95% by weight or more ofparticles satisfying the relation of 1≦R/r≦1.2 wherein R is a longdiameter and r is a short diameter of each particle, and the amount ofthe particles having long diameters R of less than 62 μm is 5% by weightor less and that of the particles having short diameters r of more than1000 μm is 5% by weight or less.

The reason why the mean particle diameter of the epoxy group-containingacrylic resin is 80 μm or more and the particles having long diameters Rof less than 62 μm are present in an amount of 5% by weight or less isthat fine powder does not fly about during the preparation of a coatingcomposition, and the deterioration of a working environment and thedecline in a yield of the coating composition can be prevented.Furthermore, the reason why the mean particle diameter is 800 μm or lessand the particles having short diameters r of more than 1000 μm arepresent in an amount of 5% by weight or less is that the resin and acrosslinking agent can be uniformly mixed during a melting/kneading stepin the preparation of the powder coating composition, so that thehardness of an obtained coating film can be improved. It is morepreferable to satisfy conditions that the mean diameter is in the rangeof 100 to 700 μm; the amount of the particles having long diameters R ofless than 62 μm is 2% by weight or less; and that of the particleshaving short diameters r of more than 1000 μm is 3% by weight or less.

Furthermore, the epoxy group-containing acrylic resin of the presentinvention is required to contain 95% by weight or more of the particlessatisfying the relation of 1≦R/r≦1.2. Because of this, the resinparticles are scarcely ground during transportation and the like, sothat fine powder which impairs handling properties is scarcelygenerated. In addition, the fluidity of the resin particles in amanufacturing apparatus is good during the preparation of the powdercoating composition, and the feed of the resin can be smoothly carriedout in the course of the melting/kneading of the resin and a curingagent, which leads to the improvement of workability. Moreover, theresin particles satisfying the relation of 1≦R/r≦1.2 are preferablypresent in an amount of 98% by weight or more.

Furthermore, the amount of volatile components contained in the epoxygroup-containing acrylic resin is required to be 1000 ppm or less. Thereason why the amount of the volatile components is 1000 ppm or less isthat the problem of an odor due to the remaining volatile components isnot present in a melting/kneading step at a high temperature requiredfor the preparation of the coating composition, and the generation ofpinholes in a formed coating film can be reduced. The amount of thevolatile components is more preferably 700 ppm or less. The volatilecomponents referred to in the present invention mainly comprise theremaining monomers for use in the polymerization.

The weight-average molecular weight of the epoxy group-containingacrylic resin is in the range of 3000 to 20000. The reason why theweight-average molecular weight is 3000 or more is that a sufficientcoating film hardness can be obtained, and the reason why it is 20000 orless is that the surface of the coating film is excellent in smoothness.The weight-average molecular weight of the epoxy group-containingacrylic resin is more preferably in the range of 5000 to 15000, mostpreferably 6000 to 12000.

The epoxy group-containing acrylic resin of the present invention can beobtained by copolymerizing a vinyl monomer mixture comprising 10 to 60%by weight of alkyl ester(s) of (meth)acrylic acid, 10 to 60% by weightof epoxy group-containing vinyl monomer(s) and other vinyl monomer(s).

Examples of the alkyl ester of (meth)acrylic acid which can be usedherein include methyl (meth)acrylate, ethyl (meth)acrylate, propyl(meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate,tert-butyl (meth)acrylate, sec-butyl (meth)acrylate, n-pentyl(meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate,2-ethylhexyl (meth)acrylate, stearyl (meth)acrylate and cyclohexyl(meth)acrylate. They can be used singly or in a combination of two ormore thereof in compliance with a use purpose. The alkyl ester of(meth)acrylic acid is preferably used in an amount of 10 to 60% byweight in the vinyl monomer mixture.

Examples of the above-mentioned epoxy group-containing vinyl monomerinclude glycidyl (meth)acrylate, methylglycidyl (meth)acrylate,3,4-epoxycyclohexylmethyl (meth)acrylate and allyl glycidyl ether. Theycan be used singly or in a combination of two or more thereof incompliance with a use purpose. Above all, glycidyl (meth)acrylate ispreferable. The epoxy group-containing vinyl monomer is preferably usedin an amount of 10 to 60% by weight in the vinyl monomer mixture and insuch an amount that an epoxy equivalent of the obtained acrylic resinmay be within the range of 350 to 1200 g/eq. This is because if theepoxy equivalent is 350 g/eq. or more, the obtained coating film isexcellent in smoothness and storage stability, and if it is 1200 g/eq.or less, the obtained coating film is excellent in hardness and solventresistance.

Incidentally, the epoxy equivalent referred to herein is a gram numberof the resin necessary to obtain 1 mol of the epoxy group.

Furthermore, examples of the above-mentioned other vinyl monomer includehydroxyalkyl esters of (meth)acrylic acid such as 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate and 4-hydroxybutyl(meth)acrylate; styrene monomers such as styrene, o-methylstyrene,m-methylstyrene, p-methylstyrene, α-methylstyrene, p-ethylstyrene,2,4-dimethylstyrene, p-n-butylstyrene, p-t-butylstyrene,p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene,p-n-dodecylstyrene, p-phenylstyrene and 3,4-dicyclohexylstyrene; anddialkyl esters of unsaturated aliphatic dibasic acids such as dimethylmaleate, diethyl maleate, dibutyl maleate, dimethyl fumarate, diethylfumarate and dibutyl fumarate. They can be used singly or in acombination of two or more thereof in compliance with a use purpose.

Above all, the styrene monomers and the hydroxyalkyl esters of(meth)acrylic acid are preferable, and the styrene monomers areparticularly preferable, and among the styrene monomers, styrene isparticularly preferable.

In the case that the styrene monomer is selected, this monomer ispreferably used in the range of 10 to 50% by weight in the vinyl monomermixture. This is because if the amount of the styrene monomer to be usedis 10% by weight or more, the storage stability of the obtained powdercoating composition and the smoothness of the obtained coating film areexcellent, and if that of the styrene monomer is 50% by weight or less,the impact resistance and the weathering resistance of the obtainedcoating film is excellent. The amount of the styrene monomer to be usedis more preferably in the range of 10 to 40% by weight.

Furthermore, since the hydroxyalkyl ester of (meth)acrylic acid canimprove a crosslinking acceleration effect and adhesive properties to acoating plate, the hydroxyalkyl ester is preferably used as the othervinyl monomer singly or together with the styrene monomer, and thelatter case is particularly preferable.

The hydroxyalkyl ester of (meth)acrylic acid is preferably used in therange of 0.5 to 50% by weight in the vinyl monomer mixture. This isbecause if the amount of the hydroxyalkyl ester of (meth)acrylic acid tobe used is 0.5% by weight or more, the hardness and the solventresistance of the obtained coating film, and the adhesive properties tothe coating plate are excellent. And if that of the hydroxyalkyl esterof (meth)acrylic acid is 50% by weight or less, the water resistance andthe pollution resistance of the obtained coating film are apt to beexcellent. The amount of the hydroxyalkyl ester of (meth)acrylic acid tobe used is more preferably in the range of 3 to 40% by weight.

A copolymerization ratio of the vinyl monomer constituting the epoxygroup-containing acrylic resin of the present invention is preferablysuch that the amount of the monomer to be used are within theabove-mentioned ranges and a glass transition temperature of the epoxygroup-containing acrylic resin is 40° C. or more. This is because if theglass transition temperature of the epoxy group-containing acrylic resinis 40° C. or more, the storage stability of the obtained powder coatingcomposition is good. The glass transition temperature of the epoxygroup-containing acrylic resin is more preferably in the range of 40 to70° C. The reason why the glass transition temperature is 70° C. or lessis that coating film properties, particularly impact resistance is good.

Furthermore, a copolymerization ratio and a molecular weight of theepoxy group-containing acrylic resin of the present invention arepreferably regulated such that its softening point is in the range of 80to 150° C. This is because if the softening point is 80° C. or more, thecoating film hardness is good. And if the softening point is 150° C. orless, the smoothness of the coating film is apt to be good.

As a polymerization method of the epoxy group-containing acrylic resinof the present invention, a suspension polymerization method can bepreferably used. This is because if the suspension polymerization methodis used, the particles having a specific particle diameter of thepresent invention can easily be obtained, and the productivity is high.In the case that the suspension polymerization method is applied, theresin for the powder coating composition of the present invention can beobtained only by separating a polymer solid from a dispersion mediumsuch as water by filtration after the polymerization. Moreover, when theresin is prepared by the suspension polymerization method, the resin ofthe spherical particles can be obtained. The resin of the sphericalparticles has an advantage in that even if the resin suffers from impactduring transportation and the like, the resin is scarcely ground,because stress is easily dispersed.

On the contrary, in the case that a solution polymerization method or abulk polymerization method is used, the obtained mass-like resin isrequired to be ground, and when it has been actually ground, a particlediameter distribution easily becomes broad, which makes the control ofthe particle diameter intricate. Moreover, the shape of each particle isa polyhedron having sharp tips, and therefore, there is a drawback thatwhen these particles have received the impact during the transportationor the like, the stress gathers at the tip to readily crush theparticles, so that fine powder is readily generated.

In the case that the epoxy group-containing acrylic resin of the presentinvention is obtained by the suspension polymerization method, apolymerization initiator, a chain transfer agent, a dispersant and thelike can be suitably selected and used in compliance with a use purpose.In order to obtain the resin within the above-mentioned ranges of thevolatile components and the weight-average molecular weight, an azocompound is preferably used as the polymerization initiator, and the useof both the azo compound and an organic peroxide having no benzene ringis most preferable. By using the azo compound as the polymerizationinitiator, it can be easily achieved to lower the molecular weight ofthe obtained resin, and by using both the azo compound and the organicperoxide having no benzene ring, a polymerization ratio (a reactionratio) of the obtained resin can be heightened to efficiently decreasethe amount of the remaining monomers for the polymerization. When anorganic peroxide having a benzene ring such as benzoyl peroxide is usedin the combination, benzoic acid is secondarily produced during thepolymerization, and it reacts with a simultaneously existing epoxygroup, whereby the ring of the epoxy group is opened and crosslinked,and in consequence, the molecular weight of the obtained polymer isincreased. In addition, this reaction consumes the epoxy group, wherebythe crosslinkage between the acrylic resin and the curing agent duringthe formation of the coating film is insufficient, so that thesmoothness and the hardness of the coating film deteriorate on occasion.For these reasons, as the organic peroxide, that having no benzene ringis preferable.

No particular restriction is put on a kind of azo compound which can beused as the polymerization initiator, so long as it is the azo compoundwhich can be used as the polymerization initiator for the usualsuspension polymerization, but it is more preferable to use both of oneor more of the azo compounds having a 10-hour half-life temperature ofless than 60° C. and one or more of the azo compounds having a 10-hourhalf-life temperature of 60° C. or more. This is because the use of theazo compound having a 10-hour half-life temperature of less than 60° C.can lower the molecular weight of the obtained polymer. And the use ofthe azo compound having a 10-hour half-life temperature of more than 60°C. as an auxiliary polymerization initiator can heighten thepolymerization ratio (the reaction ratio) of the obtained resin todecrease the amount of the remaining monomers after the polymerization.

As the azo compounds having a 10-hour half-life temperature of less than60° C., compounds having 10-hour half-life temperatures of not less than30° C. and less than 60° C. are preferable, and compounds having 10-hourhalf-life temperatures of not less than 40° C. and less than 60° C. aremore preferable. One example of such a compound is2,2′-azobis-2,4-dimethylvaleronitrile (52° C.) (the figure in theparentheses represents the 10-hour half-life temperature, and the sameshall apply hereinafter).

As the azo compounds having a 10-hour half-life temperature of 60° C. ormore, compounds having 10-hour half-life temperatures of 60° C. to 120°C. are preferable, and compounds having 10-hour half-life temperaturesof 60° C. to 100° C. are more preferable. Examples of such compoundsinclude 2,2′-azobisisobutyronitrile (65° C.),2,2′-azobis-2-methylbutyronitrile (67° C.) and1,1′-azobis-1-cyclohexanecarbonitrile (87° C.).

The amount of the azo compound to be used is preferably in the range of0.1 to 10 parts by weight based on 100 parts by weight of the vinylmonomer mixture. This is because if the amount of the azo compound is0.1 part by weight or more, the molecular weight of the obtained epoxygroup-containing acrylic resin can be sufficiently lowered, whereby thepowder coating composition having the excellent film coating smoothnesscan be obtained. And the reason why the amount of the azo compound is 10parts by weight or less is that even if the azo compound is used in anamount of more than 10 parts by weight, the effect of lowering themolecular weight is not observed any more. The amount of the azocompound to be used is more preferably in the range of 0.5 to 10 partsby weight.

Furthermore, the organic peroxide having no benzene ring which can beused together with the above-mentioned azo compound preferably has a10-hour half-life temperature of 120° C. or less. This is because theuse of the organic peroxide having a 10-hour half-life temperature of120° C. or less can achieve further reduction of the amount of theremaining monomers. Practically, the organic peroxide having a 10-hourhalf-life temperature of 60° C. or more is preferable, and the organicperoxide having a 10-hour half-life temperature of 80° C. or more ismore preferable.

Examples of the organic peroxide having no benzene ring which can beused in the present invention include t-butylperoxyisopropyl carbonate(98° C.) (the figure in the parentheses represents the 10-hour half-lifetemperature, and the same shall apply hereinafter),t-butylperoxy-2-ethylhexanoate (73°) and lauroyl peroxide (62° C.). Theamount of the organic peroxide to be used is preferably in the range of0.1 to 5 parts by weight based on 100 parts by weight of the vinylmonomer mixture. This is because if the amount of the organic peroxideis 0.1 part by weight or more, the effect of reducing the amount of theremaining monomers is sufficient. And the reason why the amount of theorganic peroxide is 5 parts by weight or less is that even if theorganic peroxide is used in an amount of more than 5 parts by weight,the amount of the remaining monomers does not change any more. Theamount of the organic peroxide is more preferably in the range of 0.3 to5 parts by weight.

In order to obtain the resin for the powder coating composition withinthe above-mentioned ranges of the volatile components and theweight-average molecular weight, the chain transfer agent can be usedtogether with the above-mentioned polymerization initiator. Examples ofthe chain transfer agent which can be used in the present inventioninclude α-methylstyrene dimer, terpinolene(1-methyl-4-isopropylidene-1-cyclohexene), n-octyl mercaptan, n-dodecylmercaptan and 2-ethylhexyl thioglycolate. They can be used singly or ina combination of two or more thereof in compliance with a use purpose.Above all, α-methylstyrene dimer and terpinolene are preferable.Particularly, the use of terpinolene as the chain transfer agent permitsobtaining the resin for the powder coating composition in which themolecular weight of the polymer is lowered and so the smoothness of thecoating film is excellent, and the odor of the obtained resin can bereduced.

Here, the amount of terpinolene to be used is preferably in the range of0.1 to 5 parts by weight based on 100 parts by weight of the vinylmonomer mixture. This is because if the amount of terpinolene is 0.1 ormore, the effect of terpinolene as the chain transfer agent can besufficiently exerted, and therefore the increase in the molecular weightof the obtained polymer can be prevented and the smoothness of thecoating film can be improved. On the other hand, if terpinolene is usedin an excessive amount, a polymerization rate deteriorates, and theamount of the remaining monomers tends to increase, so that theproductivity declines and the problem of the odor due to the remainingmonomers may arise during the melting/kneading step. However, when theamount of terpinolene is set to 5 parts by weight or less, theseproblems can be prevented. The amount of terpinolene is more preferablyin the range of 0.2 to 3 parts by weight.

In the case that the chain transfer agent other than terpinolene is usedin the suspension polymerization, the amount of the chain transfer agentto be added is preferably in the range of 0.1 to 10 parts by weightbased on 100 parts by weight of the vinyl monomer mixture. When theamount of the chain transfer agent is 0.1 part by weight or more, theeffect of the chain transfer agent can sufficiently appear, but when theamount thereof is too much, the polymerization rate deteriorates and theamount of the remaining monomers increases as in the case ofterpinolene. In practice, the amount of the chain transfer agent ispreferably 10 parts by weight or less, more preferably in the range of0.5 to 8 parts by weight.

However, in the case that a mercaptan is used as the chain transferagent to be used together, the problem of the odor of the resin mayarise, and the weathering resistance of the coating film also tends tobe poor. Accordingly, the amount of the mercaptan to be added ispreferably 0.2 part by weight or less, and in this case, the adjustmentof the molecular weight is suitably carried out by using another chaintransfer agent together.

Examples of a dispersant which can be used for the suspensionpolymerization in the present invention include polyvinyl alcohol,alkali metal salts of homopolymers and copolymers of (meth)acrylic acid,carboxymethyl cellulose, gelatin, starch, barium sulfate, calciumsulfate, calcium carbonate, magnesium carbonate and calcium phosphate.The dispersant is preferably used in the range of 0.01 to 5 parts byweight based on 100 parts by weight of water. This is because if theamount of the dispersant is 0.01 part by weight or more, the stabilityimprovement effect of the suspension polymerization can be obtained. Andif it is 5 parts by weight or less, the particle diameter of the formedparticles after the polymerization becomes not too fine, so that a wastewater can be prevented from being polluted therewith, and theproductivity is high. The amount of the dispersant to be used is morepreferably in the range of 0.05 to 2 parts by weight. If necessary, adispersing agent such as sodium chloride, potassium chloride, sodiumsulfate, potassium sulfate and manganese sulfate can also be usedtogether with the above-mentioned dispersant.

The suspension polymerization can be carried out by adding, to the vinylmonomer mixture, the polymerization initiator, the chain transfer agent,the dispersant and if necessary, the dispersing agent together withwater in an amount preferably 1 to 10 times, more preferably 1.3 to 4times as much as the vinyl monomer mixture, heating the mixture up to apredetermined polymerization temperature at a predetermined rate, andthen continuing the heating until completion of the polymerization.

Furthermore, in the present invention, the suspension polymerization isdone under conditions that a polymerization temperature is 80° C. ormore and preferably 180° C. or less and a polymerization pressure is 2kg/cm² or more and preferably 10 kg/cm² or less. The reason why thepolymerization temperature is 80° C. or more and the polymerizationpressure is 2 kg/cm² or more is that the molecular weight of theobtained epoxy group-containing acrylic resin can be sufficientlylowered and the resin for the powder coating composition which isexcellent in the smoothness of the coating film can be obtained. Thepolymerization temperature is more preferably 100° C. or more and 150°C. or less, and the polymerization pressure is more preferably 3 kg/cm²or more and 8 kg/cm² or less.

The volatile components in the epoxy group-containing acrylic resinobtained by the above-mentioned suspension polymerization can be easilyreduced to 1000 ppm or less by carrying out a removal operation of thevolatile components. These volatile components, particularly theremaining monomers can be removed by heating the system at the end ofthe polymerization to distill off the monomers, or distilling thesuspension after the polymerization to distill off the remainingmonomers. In this case, it is not always necessary to distill off all ofthe dispersion medium comprising water and the like, and by distillingoff a part of the dispersion medium, most of the remaining monomers canbe simultaneously removed. As the removal operation of the volatilecomponents, a method of distilling the suspension after thepolymerization under atmospheric pressure or reduced pressure isparticularly preferable.

In the case that the epoxy group-containing acrylic resin of the presentinvention is obtained by the suspension polymerization, the particlediameter of the resin depends on conditions regarding a reactionapparatus such as a reaction vessel and the shape of stirring blades,and polymerization conditions such as a stirring rotational frequency,kinds and amounts of dispersant and dispersing agent, and a ratiobetween the monomers and water. In the present invention, theseconditions are taken into consideration to decide a manufacturingprescription, whereby the epoxy group-containing acrylic resin having adesired shape intended by the present invention can be obtained.

The epoxy group-containing acrylic resin of the present invention can beused as the coating composition after it has been blended with a curingagent. Examples of the usable curing agent include polyvalent carboxylicacids and polyvalent phenols. The amount of the curing agent to be usedis within such a range that an equivalent ratio of the epoxy group ofthe epoxy group-containing acrylic resin to a functional group such as acarboxyl group or a phenolic hydroxyl group of the curing agent is inthe range of 1/2 to 2/1, preferably 1/1. The reason why the equivalentratio is 1/2 or more is that the coating film having a sufficient waterresistance can be obtained, and on the other hand, the reason why it is2/1 or less is that a sufficient solvent resistance can be obtained.

The powder coating composition comprising the epoxy group-containingacrylic resin of the present invention can be used as the coatingcomposition for various materials to be coated such as metals, glassesand heat-resistant plastics.

A baking temperature of the material to be coated can be suitablyselected in accordance with a kind of the material to be coated and ause purpose, but for example, the baking temperature is in the range of120 to 250° C., and a baking time is in the range of 5 to 30 minutes.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention will be described in more detail withreference to examples. In examples and comparative examples, part(s)means part(s) by weight.

Various physical properties in the examples and the comparative exampleswere evaluated by the following procedures.

(1) Mean particle diameter:

In accordance with a dry sieve analysis method stipulated by JIS K-0069,a wire sieve having an inner diameter of 75 mm stipulated by JIS Z-8801was used, and 5 g of a sample was put into a set sieve in which wiresieves having mesh openings of 1000 μm, 710 μm, 500 μm, 355 μm, 250 μm,150 μm and 75 μm, and a pan were stacked in this order from the top. Alid was then put on the set sieve, followed by shaking, to determine theresidues of the sample on the respective sieves. On the basis of thedisplay of an integrated distribution, the mesh opening through 50% byweight of the resin was passed was determined.

(2) Ratio of the resin having long diameters of less than 62 μm and thathaving short diameters of more than 1000 μm:

In accordance with a dry sieve analysis method stipulated by JIS K-0069,a wire sieve having an inner diameter of 75 mm stipulated by JIS Z-8801was used, and 5 g of a sample was put into a set sieve in which wiresieves having mesh openings of 1000 μm and 62 μm, and a pan were stackedin this order from the top. A lid was then put on the set sieve,followed by shaking, and a ratio of the resin on the pan was regarded asa ratio of the resin having long diameters of less than 62 μm, a weightratio of the residue on the wire sieve of 1000 μm was regarded as aratio of the resin having short diameters of more than 1000 μm.

(3) Shape of particles:

The shape of randomly selected 100 particles of the resin was observedby the use of a microscope to measure a short diameter r and a longdiameter R of each resin particle, and an existing ratio of the resinparticles in which a ratio R/r was within the range of 1≦R/r≦1.2 wasdetermined on the basis of the number of the resin particles within theabove-mentioned range.

(4) Glass transition temperature:

A sample was heated up to 100° C. and melt quench was then carried out,and an extrapolation glass transition start temperature was measured bya DSC method (a differential scanning calorie measurement method,temperature rise rate=10° C./min). This extrapolation glass transitionstart temperature was employed as the glass transition temperature.

(5) Softening temperature:

Measurement was made by the use of a flow tester (CFT-500 model, made byShimadzu Seisakusho Ltd.) under conditions of a temperature rise rate=3°C./min, a load=30 kgf, a nozzle diameter=1 mmΦ (diameter), a nozzlelength=10 mm, the sectional area of a plunger=1 cm² and a sampleamount=1 g, and a temperature at which ½ of the sample flowed out wasemployed as the softening temperature.

(6) Weight-average molecular weight:

The weight-average molecular weight was measured by a gel permeationchromatography (GPC) method. A sample was a THF solution obtained bydissolving 0.4 part by weight of the resin in 100 parts by weight oftetrahydrofuran. This sample was filtered through a PTFE film having apore diameter of 0.5 μm (Myshoridisc H-25-5, made by Toso Co., Ltd.),and measurement was then made by HCL-8020 (made by Toso Co., Ltd.)constituted of three TSKgel/GMH_(HL) columns (made by Toso Co., Ltd.).By the use of a calibration curve ofF2000/F700/F288/F128/F80/F40/F20/F2/A1000 (polystyrene made by Toso Co.,Ltd.) and a styrene monomer, the weight-average molecular weight wasdetermined in terms of polystyrene. In this case, the measurement wasmade at 38° C., and RI was used as a detector.

(7) Amount of volatile components:

Remaining monomers in the resin were measured by gas chromatography(GC-8A, made by Shimadzu Seisakusho Ltd.), and the thus measured totalamount was regarded as the amount of the volatile components.

(8) Epoxy equivalent:

The epoxy equivalent was measured in accordance with JIS K-7236 by anindicator titration method. A sample having 0.0006 to 0.0009 epoxyequivalent was dissolved in 10 ml of chloroform and 20 ml of aceticacid. Furthermore, to this solution, 10 ml of an acetic acid solution oftetraethylammonium bromide was added, and titration is then carried outwith a 0.1 N acetic acid perchlorate solution by the use of crystalviolet as an indicator. On the basis of the amount of the consumed 0.1 Nacetic acid perchlorate solution, the epoxy equivalent was determined.

(9) Smoothness:

The smoothness was evaluated on the basis of the following standards byvisually observing the surface of each coating film:

⊚: Very good

∘: Good

Δ: Slightly bad

×: Bad

(10) Hardness:

Evaluation was made by a pencil scratching method for the coating filmin accordance with JIS K-5400, and the hardness was represented by thehardest pencil by which the coating film was not scratched.

(11) Impact resistance:

A weight of 500 g was dropped on the coating film in accordance with JISK-5400, and a maximum height at which the surface of the coating filmdid not crack was measured. The thus measured value was regarded as theimpact resistance.

(12) Odor at high-temperature melding:

In a container with a lid, 10 g of the dried resin was heated for 1 hourin an electric furnace at 100° C., and the lid was taken off. At thistime, the odor generated from the resin was evaluated on the basis ofthe following standards:

⊚: No odor, or it was so weak that the resin was at a sufficientlypractical level

∘: Slightly odorous, but the resin was at the practical level

×: Strongly odorous, and the resin was not at the practical level

(13) Pinholes:

The surface of the coating film was visually observed, and evaluationwas made on the basis of the following standards:

∘: No pinholes, and the resin was at a practical level

Δ: Some pinholes were observed, and the resin was slightly below thepractical level

×: Many pinholes were observed, and the resin was noticeably below thepractical level

(14) Adhesive properties to a coating plate:

The coating film was cut in the grid state of 1 mm² cubes in a range of1 cm², and an adhesive tape was stuck on the cut surface and then peeledoff. The state of the coating film after the peeling of the tape wasevaluated on the basis of the following standards:

⊚: Any peeling of the coating film was not observed, and the adhesiveproperties to the coating plate was good.

∘: The peeling of the coating film was scarcely observed, and the resinwas at a practical level.

Δ: The peeling of the coating film was slightly observed, and the resinwas below the practical level.

×: The coating film was peeled off all over, and the resin wasnoticeably below the practical level.

EXAMPLE 1

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of polyvinyl alcohol, the 4% aqueous solution ofwhich has a viscosity of 24 cps at 20° C. (LA-18, made by The Shin-EtsuChemical Co., Ltd.), in 200 parts of deionized water was poured into areaction vessel. Next, to this reaction vessel, there was added amixture of 16 parts of styrene, 33 parts of methyl methacrylate, 21parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 5parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries) and 3 parts of α-methylstyrene dimer(Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 110° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.6 kg/cm².

Next, this suspension polymer was distilled under atmospheric pressurefor 2 hours, so that a distillate substantially comprising water wasallowed to drip out up to the amount of 20% of the fed deionized water.Thus the residual monomers were removed. After cooled to 40° C., theobtained polymer was sufficiently washed with water, and then dried toobtain an epoxy group-containing acrylic resin having a mean particlediameter of 420 μm. The thus obtained resin was free from any residualodor, and its particles had a substantially spherical shape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin and 20 parts of a curing agent(dodecanedicarboxylic acid) were molten and kneaded at a temperature of100° C. for 30 minutes, and an aluminum plate was coated with theresultant mixture in a known manner and then baked at 180° C. for 20minutes to obtain a coated plate having a coating film of 80 μm inthickness. The evaluation results about the coating film of the thusobtained coated plate are shown in Table 1. During the formation of acoating composition, any fine powder did not fly about, and theperformance of the coating film was good.

EXAMPLE 2

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of polyvinyl alcohol, the 4% aqueous solution ofwhich has a viscosity of 24 cps at 20° C. (LA-18, made by The Shin-EtsuChemical Co., Ltd.), in 200 parts of deionized water was poured into areaction vessel. Next, to this reaction vessel, there was added amixture of 38 parts of styrene, 32 parts of methyl methacrylate, 15parts of n-butyl acrylate, 15 parts of glycidyl methacrylate, 2 parts of2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by Pure ChemicalIndustries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59, made byPure Chemical Industries) and 3 parts of α-methylstyrene dimer (NofmerMSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 130° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.3 kg/cm².

Next, a distillation operation was carried out in the same manner as inExample 1 to obtain an epoxy group-containing acrylic resin having amean particle diameter of 360 μm. The thus obtained resin was free fromany residual odor, and its particles had a substantially sphericalshape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 89 parts of this resin were blended with 11 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 1. During the formation of a coating composition, anyfine powder did not fly about, and the performance of the coating filmwas good.

EXAMPLE 3

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 5parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries) and 3 parts of terpinolene(Terpinolene, made by Yasuhara Chemical Co., Ltd.), and suspensionpolymerization was then carried out at 110° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.8 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 1 to obtain an epoxy group-containing acrylic resin havinga mean particle diameter of 420 μm. The thus obtained resin was freefrom any residual odor, and its particles had a substantially sphericalshape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin were blended with 20 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 1. During the formation of a coating composition, anyfine powder did not fly about, and the performance of the coating filmwas good.

EXAMPLE 4

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 5parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries), 1 part of terpinolene (Terpinolene,made by Yasuhara Chemical Co., Ltd.), and 3 part of α-methylstyrenedimer (Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 110° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.5 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 1 to obtain an epoxy group-containing acrylic resin havinga mean particle diameter of 450 μm. The thus obtained resin was freefrom any residual odor, and its particles had a substantially sphericalshape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin were blended with 20 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 1. During the formation of a coating composition, anyfine powder did not fly about, and the performance of the coating filmwas good.

EXAMPLE 5

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 6parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries), 2 parts of t-butylperoxyisopropylcarbonate (Perbutyl I, made by Nippon Oils & Fats Co., Ltd.), 3 parts ofα-methylstyrene dimer (Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.)and 0.5 part of terpinolene (Terpinolene, made by Yasuhara Chemical Co.,Ltd.), and suspension polymerization was then carried out at 110° C. for2 hours in a closed system to obtain a suspension polymer. During thepolymerization, a pressure in the reaction vessel was 4.4 kg/cm².

Next, this suspension polymer was distilled under atmospheric pressurefor 1.5 hours, so that a distillate substantially comprising water wasallowed to drip out up to an amount of 10% of the fed deionized water.Thus, the residual monomers were removed. After cooled to 40° C., theobtained polymer was sufficiently washed with water, and then dried toobtain an epoxy group-containing acrylic resin having a mean particlediameter of 400 μm. The thus obtained resin was free from any residualodor, and its particles had a substantially spherical shape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin were blended with 20 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 1. During the formation of a coating composition, anyfine powder did not fly about, and the performance of the coating filmwas good.

EXAMPLE 6

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 38 parts of styrene, 32 parts of methyl methacrylate,15 parts of n-butyl methacrylate, 15 parts of glycidyl methacrylate, 2parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries), 2 parts of t-butylperoxyisopropylcarbonate (Perbutyl I, made by Nippon Oils & Fats Co., Ltd.) and 3 partsof α-methylstyrene dimer (Nofmer MSD, made by Nippon Oils & Fats Co.,Ltd.), and suspension polymerization was then carried out at 130° C. for2 hours in a closed system to obtain a suspension polymer. During thepolymerization, a pressure in the reaction vessel was 4.3 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain a resin for a powder coating compositioncomprising an epoxy group-containing acrylic resin having a meanparticle diameter of 370 μm. The thus obtained resin was free from anyresidual odor, and its particles had a substantially spherical shape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 89 parts of this resin were blended with 11 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 1. During the formation of a coating composition, anyfine powder did not fly about, and the performance of the coating filmwas good.

EXAMPLE 7

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate, 0.07 part of a polyvinyl alcohol, the 4% aqueous solution ofwhich has a viscosity of 25 cps at 20° C. (PA-18, made by The Shin-EtsuChemical Co., Ltd.), and 0.07 part of a polyvinyl alcohol, the 4%aqueous solution of which has a viscosity of 30 cps at 20° C. (C-17,made by The Shin-Etsu Chemical Co., Ltd.), in 180 parts of deionizedwater was poured into a reaction vessel. Next, to this reaction vessel,there was added a mixture of 16 parts of styrene, 33 parts of methylmethacrylate, 21 parts of n-butyl methacrylate, 30 parts of glycidylmethacrylate, 6 parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65,made by Pure Chemical Industries), 1 part of2,2′-azobis-2-methylbutyronitrile (V-59, made by Pure ChemicalIndustries), 2 parts of t-butylperoxyisopropyl carbonate (Perbutyl I,made by Nippon Oils & Fats Co., Ltd.), 3 parts of α-methylstyrene dimer(Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.) and 0.5 part ofterpinolene (Terpinolene, made by Yasuhara Chemical Co., Ltd.), andsuspension polymerization was then carried out at 110° C. for 3 hours ina closed system to obtain a suspension polymer. During thepolymerization, a pressure in the reaction vessel was 4.4 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain an epoxy group-containing acrylic resin. Thethus obtained resin was free from any residual odor, and its particleshad a substantially spherical shape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Furthermore, 80 parts of this resin were blended with 20 parts of acuring agent (dodecanedicarboxylic acid), and the same procedure as inExample 1 was then repeated to obtain a baked and coated plate. Theevaluation results about the coating film of the thus obtained coatedplate are shown in Table 1. During the formation of a coatingcomposition, any fine powder did not fly about, and the performance ofthe coating film was good.

EXAMPLE 8

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate, 0.07 part of a polyvinyl alcohol, the 4% aqueous solution ofwhich has a viscosity of 25 cps at 20° C. (PA-18, made by The Shin-EtsuChemical Co., Ltd.), and 0.07 part of a polyvinyl alcohol, the 4%aqueous solution of which has a viscosity of 30 cps at 20° C. (C-17,made by The Shin-Etsu Chemical Co., Ltd.), in 180 parts of deionizedwater was poured into a reaction vessel. Next, to this reaction vessel,there was added a mixture of 38 parts of styrene, 32 parts of methylmethacrylate, 15 parts of n-butyl methacrylate, 15 parts of glycidylmethacrylate, 2 parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65,made by Pure Chemical Industries), 1 part of2,2′-azobis-2-methylbutyronitrile (V-59, made by Pure ChemicalIndustries), 2 parts of t-butylperoxyisopropyl carbonate (Perbutyl I,made by Nippon Oils & Fats Co., Ltd.) and 3 parts of α-methylstyrenedimer (Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 130° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.3 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain an epoxy group-containing acrylic resin. Thethus obtained resin was free from any residual odor, and its particleshad a substantially spherical shape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Furthermore, 89 parts of this resin were blended with 11 parts of acuring agent (dodecanedicarboxylic acid), and the same procedure as inExample 1 was then repeated to obtain a baked and coated plate. Theevaluation results about a coating film of the thus obtained coatedplate are shown in Table 1. During the formation of a coatingcomposition, any fine powder did not fly about, and the performance ofthe coating film was good.

EXAMPLE 9

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate, 0.08 part of a polyvinyl alcohol, the 4% aqueous solution ofwhich has a viscosity of 25 cps at 20° C. (PA-18, made by The Shin-EtsuChemical Co., Ltd.), and 0.08 part of a polyvinyl alcohol, the 4%aqueous solution of which has a viscosity of 30 cps at 20° C. (C-17,made by The Shin-Etsu Chemical Co., Ltd.), in 180 parts of deionizedwater was poured into a reaction vessel. Next, to this reaction vessel,there was added a mixture of 16 parts of styrene, 33 parts of methylmethacrylate, 21 parts of n-butyl methacrylate, 30 parts of glycidylmethacrylate, 6 parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65,made by Pure Chemical Industries), 1 part of2,2′-azobis-2-methylbutyronitrile (V-59, made by Pure ChemicalIndustries), 2 parts of t-butylperoxyisopropyl carbonate (Perbutyl I,made by Nippon Oils & Fats Co., Ltd.), 3 parts of α-methylstyrene dimer(Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.) and 0.5 part ofterpinolene (Terpinolene, made by Yasuhara Chemical Co., Ltd.), andsuspension polymerization was then carried out at 110° C. for 3 hours ina closed system to obtain a suspension polymer. During thepolymerization, a pressure in the reaction vessel was 4.4 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain an epoxy group-containing acrylic resin. Thethus obtained resin was free from any residual odor, and its particleshad a substantially spherical shape.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Furthermore, 80 parts of this resin were blended with 20 parts of acuring agent (dodecanedicarboxylic acid), and the same procedure as inExample 1 was then repeated to obtain a baked and coated plate. Theevaluation results about a coating film of the thus obtained coatedplate are shown in Table 1. During the formation of a coatingcomposition, any fine powder did not fly about, and the performance ofthe coating film was good.

EXAMPLE 10

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate, 0.08 part of a polyvinyl alcohol, the 4% aqueous solution ofwhich has a viscosity of 25 cps at 20° C. (PA-18, made by The Shin-EtsuChemical Co., Ltd.), and 0.08 part of a polyvinyl alcohol, the 4%aqueous solution of which has a viscosity of 30 cps at 20° C. (C-17,made by The Shin-Etsu Chemical Co., Ltd.), in 180 parts of deionizedwater was poured into a reaction vessel. Next, to this reaction vessel,there was added a mixture of 38 parts of styrene, 32 parts of methylmethacrylate, 15 parts of n-butyl methacrylate, 15 parts of glycidylmethacrylate, 2 parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65,made by Pure Chemical Industries), 1 part of2,2′-azobis-2-methylbutyronitrile (V-59, made by Pure ChemicalIndustries), 2 parts of t-butylperoxyisopropyl carbonate (Perbutyl I,made by Nippon Oils & Fats Co., Ltd.) and 3 parts of α-methylstyrenedimer (Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 130° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.3 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain an epoxy group-containing acrylic resin. Thethus obtained resin was free from any residual odor, and its particleshad a substantially spherical shape.

Furthermore, 89 parts of this resin were blended with 11 parts of acuring agent (dodecanedicarboxylic acid), and the same procedure as inExample 1 was then repeated to obtain a baked and coated plate. Theevaluation results about a coating film of the thus obtained coatedplate are shown in Table 1. During the formation of a coatingcomposition, any fine powder did not fly about, and the performance ofthe coating film was good.

EXAMPLE 11

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 5parts of lauroyl peroxide, 1 part of t-butylperoxy-2-ethylhexanoate(Perbutyl O, made by Nippon Oils & Fats Co., Ltd.), 2 parts oft-butylperoxyisopropyl carbonate (Perbutyl I, made by Nippon Oils & FatsCo., Ltd.), 3 parts of α-methylstyrene dimer (Nofmer MSD, made by NipponOils & Fats Co., Ltd.) and 0.5 part of terpinolene (Terpinolene, made byYasuhara Chemical Co., Ltd.), and suspension polymerization was thencarried out at 120° C. for 2 hours in a closed system to obtain asuspension polymer. During the polymerization, a pressure in thereaction vessel was 5.2 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain an epoxy group-containing acrylic resin havinga mean particle diameter of 430 μm. The thus obtained resin was freefrom any residual odor.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin were blended with 20 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about a coating film of the thus obtained coated plate are shownin Table 1. During the formation of a coating composition, any finepowder did not fly about, and the performance of the coating film wassubstantially good.

EXAMPLE 12

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 6parts of benzoyl peroxide, 1 part of t-butyl peroxybenzoate (Perbutyl Z,made by Nippon Oils & Fats Co., Ltd.), 3 parts of α-methylstyrene dimer(Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.) and 2 parts ofterpinolene (Terpinolene, made by Yasuhara Chemical Co., Ltd.), andsuspension polymerization was then carried out at 130° C. for 2 hours ina closed system to obtain a suspension polymer. During thepolymerization, a pressure in the reaction vessel was 4.2 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain a resin for a powder coating compositioncomprising an epoxy group-containing acrylic resin having a meanparticle diameter of 450 μm. The thus obtained resin was free from anyresidual odor.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin were blended with 20 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about a coating film of the thus obtained coated plate are shownin Table 1. During the formation of a coating composition, any finepowder did not fly about, and the performance of the coating film wassubstantially good.

EXAMPLE 13

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 30 parts of styrene, 10 parts of 2-hydroxypropylmethacrylate, 30 parts of iso-butyl methacrylate, 30 parts of glycidylmethacrylate, 4 parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65,made by Pure Chemical Industries), 1 part of2,2′-azobis-2-methylbutyronitrile (V-59, made by Pure ChemicalIndustries), 2 parts of t-butylperoxyisopropyl carbonate (Perbutyl I,made by Nippon Oils & Fats Co., Ltd.), 3 parts of α-methylstyrene dimer(Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.) and 0.5 part ofterpinolene (Terpinolene, made by Yasuhara Chemical Co., Ltd.), andsuspension polymerization was then carried out at 110° C. for 3 hours ina closed system to obtain a suspension polymer. During thepolymerization, a pressure in the reaction vessel was 4.2 kg/cm².

Afterward, a distillation operation was carried out in the same manneras in Example 5 to obtain a resin for a powder coating compositioncomprising an epoxy group-containing acrylic resin having a meanparticle diameter of 450 μm. The thus obtained resin was free from anyresidual odor.

Table 1 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin and 20 parts of a curing agent(dodecanedicarboxylic acid) were molten and kneaded at a temperature of100° C. for 30 minutes, and an aluminum plate was coated with theresultant mixture in a known manner and then baked at 170° C. for 20minutes to obtain a coated plate having a coating film of 80 μm inthickness. The evaluation results about the coating film of the thusobtained coated plate are shown in Table 1. During the formation of acoating composition, any fine powder did not fly about, and theperformance of the coating film was good.

Comparative Example 1

A mixture of 40 parts of styrene, 20 parts of methyl methacrylate, 10parts of n-butyl acrylate, 10 parts of dibutyl fumarate, 20 parts ofglycidyl methacrylate, 2 parts of azobisisobutylonitrile (V-60, made byPure Chemical Industries) and 1 part of benzoyl peroxide was addeddropwise over 4 hours to a mixture of 70 parts of toluene and 30 partsof n-butanol which is heated to 100° C. The reaction was further carriedout for 10 hours.

Next, from the resultant reaction product, the solvent was removed at200° C. under a pressure of 1 mmHg until the solvent was not dripped outany more, and the reaction product was then ground to obtain an acrylicresin having a mean particle diameter of 800 μm. The shape of theparticles of the thus obtained resin was a polyhedron having sharp tips.

Table 2 shows the evaluation results of the characteristics of theobtained resin. The distribution of these particles were in apolydisperse state, and the resin having particle diameters of more than1000 μm was present in a ratio of 42.2%, and the resin having particlediameters of less than 62 μm was present in a ratio of 1.7%.

Next, 86 parts of this resin were blended with 14 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 2. The coating film was poor in smoothness, and duringthe formation of the coating film, some odor was observed.

Comparative Example 2

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 4parts of benzoyl peroxide, 1 part of t-butyl peroxybenzoate (PerbutylZ), made by Nippon Oils & Fats Co., Ltd.) and 3 parts of α-methylstyrenedimer (Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 130° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.7 kg/cm².

Next, this suspension polymer was cooled to 40° C., and the obtainedpolymer was sufficiently washed with water and then dried to obtain anacrylic resin having a mean particle diameter of 480 μm. The thusobtained resin was free from any residual odor.

Table 2 shows the evaluation results of the characteristics of theobtained resin.

Next, the same procedure as in Example 1 was then repeated using theresin to obtain a baked and coated plate. The evaluation results aboutthe coating film of the thus obtained coated plate are shown in Table 2.The coating film was poor in smoothness and hardness.

Comparative Example 3

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 1part of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries) and 1 part of 2-ethylhexylthioglycolate (OTG, made by Yodogawa Kagaku Kogyo Co., Ltd.), andsuspension polymerization was then carried out at 75° C. for 2 hours toobtain a suspension polymer.

Next, after this suspension polymer was cooled to 40° C., the obtainedpolymer was sufficiently washed with water and then dried to obtain anacrylic resin having a mean particle diameter of 570 μm. During thewashing of the polymer, a strong odor of the OTG was given off, and inthe dried resin, the strong odor of the OTG was left.

Table 2 shows the evaluation results of the characteristics of theobtained resin.

Next, the same procedure as in Example 1 was then repeated using theresin to obtain a baked and coated plate. The evaluation results aboutthe coating film of the thus obtained coated plate are shown in Table 2.The coating film was poor in smoothness and impact resistance, andduring the formation of the coating film, the odor was given off.

Comparative Example 4

A solution obtained by sufficiently dissolving 0.005 part of manganesesulfate and 0.225 part of a polyvinyl alcohol, the 4% aqueous solutionof which has a viscosity of 24 cps at 20° C. (LA-18, made by TheShin-Etsu Chemical Co., Ltd.), in 200 parts of deionized water waspoured into a reaction vessel. Next, to this reaction vessel, there wasadded a mixture of 16 parts of styrene, 33 parts of methyl methacrylate,21 parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 5parts of 2,2′-azobis-2,4-dimethylvaleronitrile (V-65, made by PureChemical Industries), 1 part of 2,2′-azobis-2-methylbutyronitrile (V-59,made by Pure Chemical Industries) and 3 parts of α-methylstyrene dimer(Nofmer MSD, made by Nippon Oils & Fats Co., Ltd.), and suspensionpolymerization was then carried out at 110° C. for 2 hours in a closedsystem to obtain a suspension polymer. During the polymerization, apressure in the reaction vessel was 4.6 kg/cm².

Next, this suspension polymer was cooled to 40° C., and the obtainedpolymer was sufficiently washed with water and then dried to obtain anacrylic resin having a mean particle diameter of 420 μm. During thewashing of the polymer, an odor of the monomers was given off, and inthe dried resin, the odor of the monomers was left.

Table 2 shows the evaluation results of the characteristics of theobtained resin.

Next, 80 parts of this resin were blended with 20 parts of a curingagent (dodecanedicarboxylic acid), and the same procedure as in Example1 was then repeated to obtain a baked and coated plate. The evaluationresults about the coating film of the thus obtained coated plate areshown in Table 2. In the coating film, pinholes were formed, and itssmoothness was poor, and during the formation of the coating film, anodor was given off.

Comparative Example 5

A mixture of 16 parts of styrene, 33 parts of methyl methacrylate, 21parts of n-butyl methacrylate, 30 parts of glycidyl methacrylate, 4parts of azobisisobutylonitrile (V-60, made by Pure Chemical Industries)and 1 part of benzoyl peroxide was added dropwise over 4 hours to amixture of 70 parts of toluene and 30 parts of n-butanol which is heatedto 130° C. The reaction was further carried out for 10 hours.

Next, from the resultant reaction product, the solvent was removed at200° C. under a pressure of 1 mmHg until the solvent was not dripped outany more. The reaction product was then ground into 3000 μm or less toobtain an acrylic resin. The shape of the particles of the thus obtainedresin was a polyhedron having sharp tips.

Table 2 shows the evaluation results of the characteristics of theobtained resin.

The distribution of these particles were in a polydisperse state, andthe resin having particle diameters of more than 1000 μm was present ina ratio of 33.4%, and the resin having particle diameters of less than62 μm was present in a ratio of 1.2%.

Furthermore, 80 parts of this resin were blended with 20 parts of acuring agent (dodecanedicarboxylic acid), and the same procedure as inExample 1 was then repeated to obtain a baked and coated plate. Theevaluation results about the coating film of the thus obtained coatedplate are shown in Table 2. During the formation of a coatingcomposition, fine powder flew about, and the obtained coating film waspoor in hardness and smoothness.

TABLE 1 Example 1 2 3 4 5 6 7 Characteristics Mean particle diameter(μm) 420 450 420 450 400 370 390 of resin Ratio of particles having longdiameters 1.5 1.4 1.2 1.2 1.3 1.6 0.1 of less than 62 μm (wt %) Ratio ofparticles having short diameters 2.3 2.4 2.4 2.5 2 1.8 1.2 of more than1000 μm (wt %) Ratio of resin particles of 1 ≦ R/r ≦ 1.2 (%) 99.2 99.499.5 99.3 99.4 99.4 99.6 Glass transition temperature (° C.) 54 52 50 5050 52 50 Softening temperature (° C.) 117 107 112 110 117 107 117Weight-average molecular weight (Mw) 8500 10500 7900 7800 8400 106008400 Amount of volatile components (ppm) 480 570 670 580 610 560 610Epoxy equivalent (g/eq.) 550 1020 557 555 564 1031 562 CharacteristicsSmoothness ⊚ ◯ ⊚ ⊚ ⊚ ◯ ⊚ of coating Hardness HB HB HB HB HB HB HB filmImpact resistance (cm) 30 20 30 30 30 20 30 Odor during high-temperaturemelting ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ Pinholes ◯ ◯ ◯ ◯ ◯ ◯ ◯ Adhesive properties to acoated plate ◯ ◯ ◯ ◯ ◯ ◯ ◯ Example 8 9 10 11 12 13 Characteristics Meanparticle diameter (μm) 370 330 310 430 450 450 of resin Ratio ofparticles having a long diameter of less than 0.1 0.1 0.2 1.3 1.1 1.8 62μm (wt %) Ratio of particles having a short diameter of more than 1.41.1 1.2 1.9 2.2 1.8 1000 μm (wt %) Ratio of resin particles of 1 ≦ R/r ≦1.2 (%) 99.6 99.6 99.7 99.8 99.9 99.4 Glass transition temperature (°C.) 52 50 52 60 58 50 Softening temperature (° C.) 107 117 107 126 123112 Weight-average molecular weight (Mw) 10600 8400 10600 15900 1280010000 Amount of volatile components (ppm) 560 610 560 500 380 680 Epoxyequivalent (g/eq.) 1035 567 1025 580 1370 560 Characteristics Smoothness◯ ⊚ ◯ Δ Δ ◯ of coating film Hardness HB HB HB HB B H Impact resistance(cm) 20 30 20 20 30 30 Odor during high-temperature melting ⊚ ◯ ⊚ ⊚ ⊚ ◯Pinholes ◯ ◯ ◯ ◯ ◯ ◯ Adhesive properties to a coated plate ◯ ◯ ◯ ◯ Δ ⊚

TABLE 2 Comparative Example 1 2 3 4 5 Characteristics Mean particlediameter (μm) 800 480 570 420 660 of resin Ratio of particles having along diameter 1.7 1 1.2 0.9 1.2 of less than 62 μm (wt %) Ratio ofparticles having a short diameter 42.2 1.9 2.8 1.8 33.4 of more than1000 μm (wt %) Ratio of resin particles of 1 ≦ R/r ≦ 1.2 (%) 15.3 99.899.5 99.5 19.5 Glass transition temperature (° C.) 47 60 62 50 47Softening temperature (° C.) 114 129 131 112 114 Weight-averagemolecular weight (Mw) 32000 21000 23000 8500 7000 Amount of volatilecomponents (ppm) 2380 340 2220 3840 1530 Epoxy equivalent (g/eq.) 9221280 550 557 640 Characteristics Smoothness × Δ × × ◯ of coating filmHardness HB B HB HB B Impact resistance (cm) 20 20 10 20 15 Odor duringhigh-temperature melting × ⊚ × × × Pinholes × ◯ × × Δ Adhesiveproperties to a coated plate Δ Δ Δ Δ ◯

POSSIBILITY OF INDUSTRIAL UTILIZATION

According to the present invention, there can be obtained a resin for apowder coating composition which is free from problems such as the flyof fine powder and an odor, so that the deterioration of a workingenvironment can be prevented and workability is good, and the coatingfilm has no pinholes and has excellent performances such as gloss andsmoothness.

Furthermore, by using a suspension polymerization method, the resin forthe powder coating composition of the present invention can bemanufactured with a high productivity.

What is claimed is:
 1. A powder coating composition comprising an epoxygroup-containing acrylic resin particles and a curing agent, said curingagent contained to cure the epoxy group-containing acrylic resinparticles and form a coating on a substrate, said epoxy group-containingacrylic resin particles (i) being a copolymer, having a weight-averagemolecular weight of 3000 to 20000, of a vinyl monomer mixture comprising10 to 60% by weight of alkyl ester(s) of (meth)acrylic acid, 10 to 60%by weight of epoxy group-containing vinyl monomer(s) and other vinylmonomer(s), (ii) having a mean particle diameter in the range of 80 to800 μm, (iii) including 95% by weight or more of particles satisfyingthe relation of 1≦R/r≦1.2 wherein R is a long diameter and r is a shortdiameter of each particle, (iv) comprising 5% by weight or less ofparticles having long diameters R of less than 62 μm and 5% by weight orless of particles having short diameters r of more than 1000 μm, (v)including 1000 ppm or less of volatile components, and (vi) having anepoxy equivalent in the range of 350 to 1200 g/eq.
 2. The powder coatingcomposition according to claim 1 wherein the other vinyl monomer(s) isstyrene monomer(s) and is contained in an amount of 10 to 50% by weightin the vinyl monomer mixture.
 3. The powder coating compositionaccording to claim 1 wherein said epoxy group-containing acrylic resinis prepared by a suspension polymerization method.
 4. The powder coatingcomposition according to claim 1 wherein said epoxy group-containingacrylic resin is prepared by the suspension polymerization method inwhich terpinolene is used as a chain transfer agent.
 5. The powdercoating composition according to claim 1, which is prepared bypolymerizing a monomer mixture comprising 10 to 60% by weight of alkylester(s) of (meth)acrylic acid, 10 to 60% by weight of epoxygroup-containing vinyl monomer(s), and other vinyl monomer(s) bysuspension polymerization at a polymerization temperature of 80° C. orhigher under a polymerization pressure of 2 kg/cm² or higher, followedby removing volatile components included in the suspension polymer.